A base station of a cellular wireless communication system consists of an antenna, which transmits and receives signals from mobile terminals within that cell. Often, advanced antenna arrangements are used in such systems. These consist of an array of antennas, known as a smart antenna array. Smart antenna arrays (or multi-antennas) significantly increase the capacity and coverage of cellular systems. Thus form multi-beams, which are classified depending on the applied beam forming method.
One such beam forming method is fixed beam forming (also referred to as switched beam forming). In the fixed beam forming method, there exists a set of predefined beams with fixed pointing directions that cover a specific area. Typically, each of the predefined beams serves more than one mobile terminal.
Another such method is adaptive beam forming (also referred to as steerable beam forming). In the adaptive beam forming method, the weights of the antenna array are adapted to maximize a certain number of desired criteria. For example, adaptive beam forming could be used to attenuate or eliminate interferers that arrive with Direction of Arrival (DOA) near to the desired signals. Fixed beam forming does not have the same flexibility as adaptive beam forming. However, it has other advantages, such as being easy to implement, widely available and cheap. In addition, if a sufficient number of beams are used in the fixed beam forming method, then the performance of this method can approach that of the adaptive beam forming method.
An extensive evaluation of advanced antennas in wireless networks using Wideband Code Division Multiple Access WCDMA has been well documented. An example can be found in “Downlink capacity comparison between different smart antenna concepts in a mixed service WCDMA system”, M. Ericson, A. Osseiran, J. Barta, B. Göransson and B. Hagerman, proceedings IEEE Vehicular technology conference, Fall, Atlantic City, USA, 2001, vol. 3, pp. 1528-1532.
Beam forming schemes such as Transmission Antenna Array (TxAA) have been implemented in the 3rd Generation Partnership Project (3GPP). Examples can be found in “Technical Specification Group Radio Access Network, Physical layer procedures (FDD)”, 3GPP TS 25.214, March 2006 and in “MIMO Reference Cases”, Nokia, 3GPP R1-040439, TSG-RAN WG1 #37 Meeting, May 2004.
Further to the beam forming methods, it is also necessary to employ a selection method that establishes when each mobile terminal is to receive data from the base station. The method of scheduling used can achieve multi-user diversity gain for the smart antenna system.
One such scheduling method that can be used is a deterministic scheduling scheme. In a deterministic scheduling scheme, conventional round robin scheduling is used, which schedules mobile terminals based on some certain orders or rules. The base station then transmits using the available physical resources to the scheduled mobile terminal. The round robin scheduling scheme is channel independent.
Another scheduling method that can be used is an opportunistic scheduling scheme, which applies to a system with multi-antennas and multiple users. An opportunistic scheduling scheme uses a Channel State Indicator (CSI) or other mobile terminal information. For example, the mobile terminals estimate and feedback their instantaneous downlink (DL) Signal to Noise Ratio (SNR) to the base station and the base station then transmits to the mobile terminal reporting the best quality. In other words, each mobile terminal receives data from the base station when their channel peaks. This opportunistic method can combat frequency-selective fading, which is a propagation anomaly caused by partial cancellation of a signal by itself. It occurs since the signal reaching the receiving antenna arrives by two or more paths (multi-paths), and at least one of the paths is changing (lengthening or shortening).
A description of the opportunistic and deterministic scheduling schemes can be found in “Opportunistic Beamforming with Limited Feedback”, Shahab Sanayei and Aria Nosratinia, accepted in the Asilomar Conference on Signals, Systems and Computers, Oct. 28-Nov. 1, 2005, pages 648-652.
It is also possible to counteract the effect of frequency-selective fading by applying a transmission scheme, such as Orthogonal Frequency Division Multiple Access (OFDMA) modulation. In the OFDMA scheme, the transmission bandwidth is divided into a set of narrowband resource blocks. Different resource blocks are then allocated to different mobile terminals. In other words, there exists a dynamic resource block assignment (or scheduling) that provides a flexible multi-user access scheme. The scheduling scheme used for the resource block assignment could be, for example, a proportional fair scheduling scheme, such as that disclosed in U.S. Pat. No. 6,807,426, which uses a variety of combinations of channel condition metrics and user fairness metrics for a WCDMA system. However, the proportional fair scheduling scheme can easily extend to an OFDMA system. Such a scheme will be presented here as “proportional fair scheduling in both time and frequency domain” (PFTF). PFTF is used to perform a proportional fair scheduling in time and frequency domain based on the throughput for each mobile terminal and the channel quality measurements for each resource block per mobile terminal. The PFTF scheme achieves resource fairness by providing a fair sharing of transmission time that is proportional to the past throughputs of mobile terminals over a fixed window length. The PFTF scheduling scheme is channel dependent.
A further scheduling technique can be found in EP 1224747, in which the scheduling information is attached to data blocks and the data blocks are transmitted to the desired users. A comprehensive summary of scheduling techniques can be found in “Adaptive sub-carrier and power allocation in OFDM based on maximizing utility”, Guocong Song and Ye (Geoffrey) Li, Vehicular Technology Conference, 2003, VTC 2003-Spring, The 57th IEEE Semi-annual, Volume 2, pp 22-25 Apr. 2003. Further summaries can be found in U.S. Pat. No. 6,807,426 and in “Scheduling algorithms for HS-DSCH in a WCDMA mixed traffic scenario”, M. Kazmi and N. Wiberg, International symposium on personal, indoor and mobile radio communications (PIMRC), Beijing, China, September 2003.
WO 03/021993 discloses a transceiver for use in a wireless access network comprising a plurality of base stations, which each transmit data to several subscribers that are directionally along the same antenna beam in consecutive bursts.
US 2005/070266 discloses a method and apparatus for reducing signal interference within a cellular radio system by altering the direction of beams within cell sectors among discrete angular positions according to a predetermined, cyclic pattern.
EP 1,528,830 discloses a communication system that includes a scheduler that assigns the same system resources for use during a simultaneous data transmission to a receiver in each of the coverage envelopes provided that the coverage envelopes do not overlap.
EP 1,184,938 discloses a load sharing system and method using a cylindrical antenna array in which antenna array components are adjusted to share loads among sectors of a cell or between cells.
However, the problem that exists is that if the pattern of the beams (beam patterns) in interfering cells is not known, then it is impossible to achieve an accurate channel quality estimation, which vastly reduces the system performance. This becomes more serious for OFDMA systems when a channel dependent scheduling scheme is employed on resource block level, since it is impossible for one cell to know the beam pattern selection of interfering cells in each resource block. It is also impossible to prevent inter-cell interference without knowing the beam patterns in interfering cells. Furthermore, when there are many mobile terminals with different beam patterns to be scheduled per slot, the implementation becomes complex and downlink (DL) and uplink (UL) signaling overhead is greater.
Improvement and simplification is therefore required for a multi-user wireless communication system that can achieve accurate channel quality estimation, reduced inter-cell interference, a simple transmit structure and reduced signaling overhead.